Power schottky diodes having local current spreading layers and methods of forming such devices

ABSTRACT

A Schottky diode includes a drift region doped with dopants having a first conductivity type, first and second blocking junctions that are doped with dopants having a second conductivity type in an upper portion of the drift region, first and second local current spreading layers doped with dopants having the first conductivity type underneath the respective first and second blocking junctions, and first and second contacts on respective lower and upper portions of the drift region. A channel is provided in the upper portion of the drift region between the first and second blocking junctions, the channel doped with dopants having the first conductivity type and a concentration of dopants in at least a first portion of the channel being lower than the concentration of dopants in the first and second local current spreading layers.

FIELD OF THE INVENTION

The present invention relates to power semiconductor devices and, moreparticularly, to power Schottky diodes.

BACKGROUND

Power semiconductor devices are used to carry large currents and supporthigh voltages. High power semiconductor devices are typically fabricatedfrom silicon carbide or gallium nitride based semiconductor materials.One widely used power semiconductor device is the Schottky diode.

Power semiconductor devices can have a lateral structure or a verticalstructure. In a device having a lateral structure, the terminals of thedevice are on the same major surface (i.e., top or bottom) of asemiconductor layer structure. In contrast, in a device having avertical structure, at least one terminal is provided on each majorsurface of the semiconductor layer structure (e.g., in a Schottky diode,the anode contact may be on the top surface of the device and thecathode contact may be on the bottom surface of the device). Thesemiconductor layer structure may or may not include an underlyingsubstrate. Herein, the term “semiconductor layer structure” refers to astructure that includes one or more semiconductor layers such assemiconductor substrates and/or semiconductor epitaxial layers.

A conventional silicon carbide power Schottky diode typically has asilicon carbide substrate having a first conductivity type (e.g., ann-type substrate), on which an epitaxial layer structure having thefirst conductivity type (e.g., n-type) is formed. This epitaxial layerstructure (which may comprise one or more separate layers) functions asa drift region of the device. The Schottky diode includes an “activeregion” that may be formed on and/or in the drift region. The activeregion acts as a main junction for blocking voltage in the reverse biasdirection and providing current flow in the forward bias direction.Typically, a plurality of Schottky diodes are formed on a wafer. EachSchottky diode typically has a “unit cell” structure in which the activeregion of the device includes a plurality of individual diodes that aredisposed in parallel to each other and that together function as asingle power Schottky diode. Each power Schottky diode will typicallyhave its own edge termination. After the wafer is fully formed andprocessed, the wafer may be diced to separate the individualedge-terminated power Schottky diodes. The portion of the wafer includedin each individually singulated device is called the substrate.

A power Schottky diode is designed to block (in the reverse blockingstate) or pass (in the forward operating state) large voltages and/orcurrents. For example, in the blocking state, a power Schottky diode maybe designed to sustain hundreds or thousands of volts of electricpotential. However, as the applied voltage approaches or passes thevoltage level that the device is designed to block, non-trivial levelsof current may begin to flow through the diode. Such current, which istypically referred to as “leakage current,” may be highly undesirable.Leakage current may begin to flow if the voltage is increased beyond thedesign voltage blocking capability of the device, which may be afunction of, among other things, the doping and thickness of the driftregion. Current leakage can also occur for other reasons, such aselectric field crowding at the edges of the active region and/or failureof an edge termination and/or the primary junction of the device. If thevoltage on the device is increased past the breakdown voltage to acritical level, the increasing electric field may result in anuncontrollable and undesirable runaway generation of charge carrierswithin the semiconductor device, leading to a condition known asavalanche breakdown.

For a vertical Schottky diode, the blocking voltage rating is typicallydetermined by the thickness and the doping concentration of the driftregion. The breakdown voltage of the device may be increased by reducingthe doping concentration of the drift region and/or by increasing thethickness of the drift region. Typically, during the design phase, adesired blocking voltage rating is selected, and then the thickness anddoping of the drift region are chosen based on the desired blockingvoltage rating. Since the drift region is the current path for thedevice in the forward “on” state, the decreased doping concentrationand/or increased thickness of the drift region may result in a higheron-state resistance for the device. Thus, there is an inherent tradeoffbetween the on-state resistance and blocking voltage.

FIG. 1 is a schematic cross-sectional diagram of a conventional powerJunction Barrier Schottky (“JBS”) diode 10. As shown in FIG. 1, the JBSdiode 10 includes a cathode contact 20, an ohmic contact layer 22, ann-type substrate 24, an n-type drift region 30, a p-type blockingjunction 40, a channel 46, a Schottky contact 42 and an anode contact44. The cathode contact 20 and the anode contact 44 may each comprise ahighly conductive metal layer. The Schottky contact 42 may comprise alayer that forms a Schottky junction with the drift region 30 and maycomprise, for example, an aluminum layer. The n-type substrate 24 maycomprise a silicon carbide substrate that is heavily doped with n-typeimpurities such as nitrogen or phosphorous. The ohmic contact layer 22may comprise a metal that forms an ohmic contact to n-type siliconcarbide so as to form an ohmic contact to the silicon carbide substrate24. The drift region 30 may comprise an epitaxially grown n-type siliconcarbide semiconductor region. The p-type blocking junction 40 may be ap-type implanted region in an upper portion of the drift region 30 thatis heavily implanted with p-type dopants. The channel 46 is positionedadjacent the p-type blocking junction 40. Current flows through thechannel 46 when the diode 10 is in its forward on-state.

SUMMARY

Pursuant to embodiments of the present invention, power Schottky diodesare provided that include a drift region having an upper portion and alower portion, at least some of the drift region doped with dopantshaving a first conductivity type. First and second blocking junctionsare provided in the upper portion of the drift region that are dopedwith dopants having a second conductivity type, the second conductivitytype being opposite the first conductivity type. First and second localcurrent spreading layers are provided underneath the respective firstand second blocking junctions, the first and second local currentspreading layers doped with dopants having the first conductivity type.A first contact is provided on the upper portion of the drift region anda second contact is provided on the lower portion of the drift regionand vertically spaced apart from the first contact. A substrate may beinterposed between the drift region and the second contact in someembodiments. A channel is provided in the upper portion of the driftregion between the first and second blocking junctions, the channeldoped with dopants having the first conductivity type and aconcentration of dopants in at least a first portion of the channelbeing lower than the concentration of dopants in the first and secondlocal current spreading layers.

In some embodiments, the first local current spreading layer may includea lateral section that extends underneath the first blocking junctionand a vertical section that extends upwardly from the lateral sectionalong a sidewall of the first blocking junction, the vertical sectioncomprising part of the channel.

In some embodiments, a width of the vertical section of the first localcurrent spreading layer may be between 0.1 and 0.75 microns.

In some embodiments, a distance between the first and second blockingjunctions may be at least 1.5 microns.

In some embodiments, a doping concentration of at least a portion of thelocal current spreading layer may exceed a doping concentration of thefirst portion of the channel by at least a factor of five.

In some embodiments, a lateral width of the first blocking junction maybe approximately equal to a lateral width of the first current spreadinglayer.

In some embodiments, the drift region may include a superjunctionstructure having alternating vertically extending regions of siliconcarbide having the respective first and second conductivity types.

In some embodiments, the drift region, the first and second blockingjunctions and the first and second local current spreading layers may besilicon carbide.

In some embodiments, first and second doping concentrations of therespective first and second local current spreading layers exceed athird doping concentration of the lower portion of the drift region anda fourth doping concentration of an upper portion of the drift regionthat is below the first and second local current spreading layers.

Pursuant to further embodiments of the present invention, power Schottkydiodes are provided that include a drift region having an upper portionand a lower portion, at least some of the drift region doped withdopants having a first conductivity type; a first blocking junction anda second blocking junction in the upper portion of the drift region, thefirst and second blocking junctions doped with dopants having a secondconductivity type, the second conductivity type being opposite the firstconductivity type; a first contact on the upper portion of the driftregion; a second contact on the lower portion of the drift region andseparated from the first contact along a vertical axis, and a channeldoped with dopants having the first conductivity type in the upperportion of the drift region between the first and second blockingjunctions, the channel having a non-uniform doping concentration along alateral cross-section thereof. A substrate may be interposed between thedrift region and the second contact in some embodiments.

In some embodiments, the Schottky diode may further include a firstlocal current spreading layer underneath the first blocking junction anda second local current spreading layer underneath the second blockingjunction, the first and second local current spreading layers doped withdopants having the first conductivity type, the concentration of dopantsin the first and second local current spreading layers being higher thanthe concentration of dopants in the drift region.

In some embodiments, a vertical section of the first local currentspreading layer may comprise a first side portion of the channel and avertical section of second local current spreading layer may comprise asecond side portion of the channel that is opposite the first side ofthe channel, the vertical sections of the first and second local currentspreading layers having a higher doping concentration than a middlesection of the channel so that the channel has the non-uniform dopingconcentration along the lateral cross-section thereof.

In some embodiments, a width of the vertical section of the first localcurrent spreading layer may be between 0.1 and 0.75 microns.

In some embodiments, a distance between the first and second blockingjunctions may be at least 1.5 microns.

In some embodiments, a doping concentration of at least a portion of thelocal current spreading layer may exceed a doping concentration of amiddle section of the channel by at least a factor of five.

In some embodiments, a lateral width of the first blocking junction maybe less than a lateral width of the first current spreading layer.

In some embodiments, the drift region may include a superjunctionstructure having alternating vertically extending regions of siliconcarbide having the respective first and second conductivity types.

In some embodiments, the drift region, the first and second blockingjunctions and the first and second local current spreading layers may besilicon carbide.

Pursuant to still further embodiments of the present invention, methodsof fabricating a power Schottky diode are provided in which a driftregion that is doped with dopants having a first conductivity type isformed on a substrate. First and second blocking junctions that aredoped with dopants having a second conductivity type are formed in theupper portion of the drift region, the second conductivity type beingopposite the first conductivity type. First and second local currentspreading layers are formed in the upper portion of the drift region,the first and second local current spreading layers doped with dopantshaving the first conductivity type. A first contact is formed on theupper portion of the drift region and a second contact is formed on thelower portion of the drift region, the second contact being verticallyspaced apart from the first contact. A substrate may be interposedbetween the drift region and the second contact in some embodiments. Achannel is provided in the upper portion of the drift region between thefirst and second blocking junctions, the channel doped with dopantshaving the first conductivity type and a concentration of dopants in atleast a first portion of the channel being lower than the concentrationof dopants in the first and second local current spreading layers. Atleast a portion of the first local current spreading layer is underneaththe first blocking junction and at least a portion of the second localcurrent spreading layer underneath the second blocking junction.

In some embodiments, the first and second blocking junctions may beformed by forming an ion implantation mask pattern on an upper surfaceof the drift region and then implanting dopants having the secondconductivity type into the upper portion of the drift region throughopenings in the ion implantation mask pattern to form the first andsecond blocking junctions.

In some embodiments, the first and second local current spreading layersmay be formed underneath the respective first and second blockingjunctions by partially etching the ion implantation mask pattern toprovide an etched ion implantation mask pattern and then implantingdopants having the first conductivity type into the upper portion of thedrift region through openings in the ion implantation mask pattern toform the first and second local current spreading layers.

In some embodiments, the dopants having the first conductivity type thatare implanted to form the first and second local current spreadinglayers are implanted to a depth deeper into the drift region than thedopants having the second conductivity type that are implanted to formthe first and second blocking junctions.

In some embodiments, the dopants implanted to form the first and secondlocal current spreading layers may be implanted using a channeled ionimplantation process.

In some embodiments, the drift region comprises 4H silicon carbide, andimplanting dopants having the first conductivity type into the upperportion of the drift region through openings in the etched ionimplantation mask pattern to form the first and second local currentspreading layers may comprise implanting the first conductivity typedopants at an angle that is within +/−1.5° of one of the <11-23>,<−1-123>, <1-213>, <−12-13>, <2-1-13> or <−2113> crystallographic axesof the drift region.

In some embodiments, the first local current spreading layer maysurround a bottom and sidewalls of the first blocking junction.

In some embodiments, the first local current spreading layer may includea portion that is within the channel.

In some embodiments, a lateral width of the portion of the first localcurrent spreading layer that is within the channel may be between 0.1and 0.75 microns.

In some embodiments, a distance between the first and second blockingjunctions may be at least 1.5 microns.

In some embodiments, a doping concentration of at least a portion of thelocal current spreading layer may exceed a doping concentration of thefirst portion of the channel by at least a factor of five.

In some embodiments, a lateral width of the first blocking junction maybe approximately equal to a lateral width of the first local currentspreading layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram of a conventional powerSchottky diode.

FIG. 2A is a graph illustrating the tradeoff between the forward voltageperformance and reverse leakage current performance in a conventionalpower Schottky diode.

FIG. 2B is a graph illustrating how it may be desirable to shift theV_(F)-I_(R) curve in order to obtain improved diode performance.

FIG. 3A is a schematic plan view of a power Schottky diode according tocertain embodiments of the present invention.

FIG. 3B is a schematic cross-sectional view taken along line 3B-3B ofFIG. 3A.

FIG. 4 is a schematic cross-sectional view of a power Schottky diodeaccording to embodiments of the present invention that has both localcurrent spreading layers and a drift region having a superjunctionstructure.

FIGS. 5A-5F are schematic cross-sectional diagrams that illustrate amethod of fabricating a power Schottky diode according to embodiments ofthe present invention.

FIG. 6 is a flowchart that illustrates a method of fabricating a powerSchottky diode having local current spreading layers according toembodiments of the present invention.

FIGS. 7A-7C are graphs that illustrate the performance of power Schottkydiodes according to embodiments of the present invention.

DETAILED DESCRIPTION

An inherent tradeoff generally exists between the forward voltage(V_(F)) of a power Schottky diode and the reverse leakage current(I_(R)) of the device. This tradeoff is illustrated in FIG. 2A, which isgraph illustrating the V_(F) versus I_(R) characteristics faced by adevice designer for a conventional power Schottky diode. As illustrated,if the designer has to, for example, maintain the forward voltage belowa specified level V_(F1), then the reverse leakage current level isessentially set at the level I_(R1) by the characteristics of the curve.

Demand exists for power Schottky diodes that exhibit both a low forwardvoltage and a low reverse leakage current. A method to obtain such adevice is to move the V_(F)-I_(R) curve of FIG. 2A downwardly and to theleft. This is shown schematically in FIG. 2B. As illustrated, if thecurve is moved in such a fashion, the forward voltage V_(F1) may beachieved while the reverse leakage current can be reduced below thevalue I_(R1) to a lower value I_(R2).

One technique that can be used to move the V_(F)-I_(R) curve of FIG. 2Adownwardly and to the left as shown in FIG. 2B is to use a drift regionthat has a so-called superjunction structure. A superjunction-type driftregion has alternating, side-by-side heavily-doped n-type and p-typeregions that are often referred to as “pillars.” The pillars may havevarious shapes, such a column shapes, bar shapes, etc. The thickness anddoping of these pillars may be controlled so that the superjunction willact like a p-n junction with low resistance and a high breakdownvoltage. Thus, the superjunction structures may reduce or avoid theconventional tradeoff between the breakdown voltage of the device andthe doping level and thickness of the drift region, allowing for a morehighly doped drift region without a corresponding reduction in thereverse breakdown voltage of the device. However, superjunction driftregions may increase the fabrication cost of the device.

Another technique for reducing the reverse leakage current of a powerSchottky diode while maintaining a desired forward voltage level is toincrease the doping in the channels (see channel 46 in FIG. 1). This maybe accomplished, for example, by more highly doping the upper portion ofthe drift region to form a current spreading layer therein prior toformation of the blocking junctions. The channels are thus formed in themore highly-doped current spreading layer. The provision of this morehighly-doped current spreading layer (and hence higher channel doping)may reduce the reverse leakage current of the device without asignificant increase in the forward voltage. However, generallyspeaking, the ideal width of the channels is a function of the dopingconcentration of the channels, with increased doping levelscorresponding to reduced channel widths. This may result in problemswhen the doping of the channels is increased beyond a certain level.

In particular, a state-of-the-art conventional silicon carbide basedpower Schottky diode that does not include a current spreading layer (insuch a device the entire drift region may be doped to a concentrationof, for example, 2×10¹⁶/cm³), may have a channel width on the order of 3microns for each channel in the device. If a current spreading layerhaving an increased dopant concentration of, for example, 5×10¹⁶/cm³ isformed in the upper portion of the drift region, the ideal width of thechannel may decrease to, for example, 1.5-2.0 microns. In order toincrease the doping of the channel to, for example, a concentration of1×10¹⁷/cm³, the lateral width of the channel needs to be reduced toperhaps 0.5 microns or less.

As the width of the channel is reduced, it may become difficult to formblocking regions and channels therebetween with sufficient accuracy toensure proper device operation. For example, the drift region istypically formed via epitaxial growth. However, in silicon carbide itmay be difficult to obtain a constant doping level during the epitaxialgrowth process, and hence variations in doping of, for example, +/−15%are common. Moreover, the oppositely doped blocking junctions aretypically formed by ion implantation, and ion implantation masks areused to shield the channels during the formation of the blockingjunctions by implantation. The ion implantation masks are typicallyformed using a photolithography process. Because the semiconductorsubstrate may be bowed, portions of the photoresist mask that are overthe channels may be exposed to light, and hence variations in the widthof the opening may be 0.1 microns or more. Given the combination of thevariation in the width of the channel and the variation in the dopingconcentration it may be difficult to consistently fabricate powerSchottky diodes that have nominal 0.5 micron channel widths.

Pursuant to embodiments of the present invention, power Schottky diodesare provided that have local current spreading layers. The local currentspreading layers may comprise a series of discontinuous, morehighly-doped regions that are formed in the upper portion of the driftregion. In some embodiments, the local current spreading layers may beformed underneath the blocking junctions. In some embodiments, the localcurrent spreading layers may also be formed in the channels along one ormore sidewalls of the blocking junctions. In such embodiments, thechannels may have a non-uniform doping concentration along a lateralcross-section thereof. The local current spreading layers may reduce theintensity of the electric field in the channels during reverse blockingoperation, which may facilitate reducing the reverse leakage current ofthe device. Moreover, since at least a portion of each channel may bemore lightly doped (e.g., doped to the same concentration as theremainder of the drift region), the ideal width of the channels may notbe narrowed (or at least not significantly narrowed). Thus, the powerSchottky diodes according to embodiments of the present invention may beconsistently fabricated in a commercial production setting due to theirincreased channel widths while maintaining a desired forward voltagelevel and exhibiting reduced reverse leakage current.

Example embodiments of the present invention will now be described withreference to FIGS. 3A-7C. It will be appreciated that features of thedifferent embodiments disclosed herein may be combined in any way toprovide many additional embodiments.

FIG. 3A is a simplified schematic plan view of a power Schottky diode100 according to embodiments of the present invention. In FIG. 3A, thetopside metallization of the power Schottky diode 100 has been omittedto illustrate the locations of the channels and the blocking junctions.FIG. 3B is a schematic cross-sectional view of the power Schottky diode100 taken along line 3B-3B of FIG. 3A. It will be appreciated that FIG.3B only illustrates portions of several unit cells of the device.

As shown in FIGS. 3A-3B, the power Schottky diode 100 includes a cathodecontact 110, an ohmic contact layer 112 on the cathode contact 110, anda semiconductor substrate 120 on the ohmic contact layer 112 oppositethe cathode contact 110. The illustrated device is a silicon carbidebased n-type Schottky diode, so the semiconductor substrate 120 may bean n-type silicon carbide semiconductor substrate 120 that is doped withn-type dopants. An n-type silicon carbide drift region 130 may be formedvia epitaxial growth on the n-type silicon carbide substrate 120. Aseries of p-type blocking junctions 140 are formed in the upper portionof the drift region 130. Channels 150 may be defined in the portions ofthe n-type drift region 130 that are between the p-type blockingjunctions 140. A Schottky contact 170 may be formed on top of theblocking junction 140 and the channels 150. An anode contact 180 may beformed on the Schottky contact 170 opposite the drift region 130.

The n-type silicon carbide semiconductor substrate 120 may comprise, forexample, a 4H silicon carbide semiconductor wafer. The substrate 120 maybe doped with n-type impurities (i.e., an n⁺ silicon carbide substrate).The impurities may comprise, for example, nitrogen and/or phosphorous.The doping concentration of the substrate 120 may be, for example,between 1×10¹⁸ atoms/cm³ and 1×10²¹ atoms/cm³. The substrate 120 may beany appropriate thickness (e.g., between 100 and 500 microns thick). Thethickness of the substrate shown in FIG. 3B is not to scale to betterillustrate the structure of the other layers in the power Schottky diode100.

The n-type silicon carbide drift region 130 may be doped during growthwith n-type dopants to a concentration of, for example, 2×10¹⁶/cm³. Inexample embodiments, the drift region 130 may be between 3 and 100microns thick.

The p-type blocking junctions 140 may be highly doped with p-typedopants (e.g., to a concentration of 5×10¹⁸/cm³), and may be formed viaion implantation in some embodiments in an upper portion of the driftregion 130. The blocking junctions 140 may reduce the electric field tohelp shield the Schottky contact 170 from the electric field when thepower Schottky diode 100 operates in the reverse blocking state. Thep-type blocking junction 140 may be formed by ionizing p-type dopantsand accelerating the ions in an ion implantation target chamber at apredetermined kinetic energy as an ion beam into the upper surface ofthe drift region 130. Based on the predetermined kinetic energy, theionized p-type dopants may penetrate into the drift region 130 to acertain depth. The channels 150 pass current in the on-state and blockvoltage in the blocking state. Current flows through the channels 150when the diode 100 is in its forward on-state.

The cathode contact 110 may comprise a highly conductive metal layersuch as a silver layer. In some embodiments, the cathode contact 110 maycomprise a multilayer metal structure such as, for example, a Ti/Ni/Agstructure. The ohmic contact layer 112 may comprise a material thatforms an ohmic contact to the substrate 120. In some embodiments, thesubstrate 120 may be partially or completely removed prior to formationof the ohmic contact layer 112 and the cathode contact 110.

The Schottky contact 170 may comprise a conductive layer that forms aSchottky junction with the silicon carbide drift region 130 and maycomprise, for example, a titanium layer or a nickel layer. The anodecontact 180 may comprise a highly conductive metal contact such as analuminum layer.

As is further shown in FIG. 3B, an n-type local current spreading layer160 may be formed to at least partially surround each p-type blockingjunction 140. Each local current spreading layer 160 may include alateral section 162 that extends laterally underneath a respective oneof the p-type blocking junctions 140, and one or more vertical sections164 that each extend along a sidewall of a p-type blocking junction 140.The vertical sections 164 comprise part of the channel 150. The localcurrent spreading layers 160 may be doped with n-type dopants at adoping concentration that is higher than the doping concentration of then-type drift region 130. In example embodiments the n-type local currentspreading layers 160 may have doping concentrations that are at leastfour times the doping concentration of the n-type drift region 130. Insome embodiments, the doping concentration of the of the n-type localcurrent spreading layers 160 may be 3-10 times the doping concentrationof the n-type drift region 130. In some embodiments, an upper portion ofthe drift region 130 that is below the local current spreading layers160 may have a doping concentration that is greater than a dopingconcentration of the lower portion of the drift region 130. In suchembodiments, the local current spreading layers 160 may have dopingconcentrations that exceed the doping concentration of the upper portionof the drift region 130 that is below the first and second local currentspreading layers 160. For example, the n-type local current spreadinglayers 160 may be doped to a concentration of 1×10¹⁷/cm³ in an exampleembodiment.

Since the local current spreading layers 160 include vertical sectionsthat extend into the channels 150, the channels 150 may have anon-uniform doping concentration along a lateral cross-section thereof.For example, referring again to FIG. 3B, along the horizontal line 152that extends laterally across the channel 150, the doping concentrationof the channel 150 varies, with the doping concentration (of n-typedopants in this example) being higher at the edge regions where the lineis within the local current spreading layers 160 and lower in the middleportion of the channel 150.

As is shown in FIG. 3A, a plurality of guard rings 190 surround theactive region of the power Schottky diode 100. The guard rings 190 maycomprise p-type trench regions that are formed via ion implantation intoan upper surface of the drift region 130. The guard rings 190 are formedoutside of the active region of the power Schottky diode 100. The guardrings 190 may extend into the drift region 130 to a depth that is aboutthe same as the depth of the blocking junctions 140. The guard rings 190may comprise edge termination structures. As known to those of skill inthe art, when power semiconductor devices are operated in the blockingstate, leakage currents may begin to flow at the edges of the activeregion as the voltage is increased. Leakage currents tend to flow inthese edge regions because electric field crowding effects at the edgeof the device may result in increased electric fields in these regions.If the voltage applied to the device is increased past the breakdownvoltage to a critical level, the increasing electric field may result inrunaway generation of charge carriers within the semiconductor device,leading to avalanche breakdown. When avalanche breakdown occurs, thecurrent increases sharply and may become uncontrollable, and anavalanche breakdown event may damage or destroy the semiconductordevice.

In order to reduce this electric field crowding (and the resultingincreased leakage currents), edge termination structures such as guardrings may be provided that surround part or all of the active region ofa power semiconductor device. These edge termination structures may bedesigned to spread the electric field out over a greater area, therebyreducing the electric field crowding. Guard rings are one known type ofedge termination structure. While FIG. 3A illustrates a powersemiconductor device 100 that uses two guard rings 190 as an edgetermination structure, it will be appreciated that more or fewer guardrings 190 may be provided, and that any appropriate edge terminationstructure may be used. For example, in other embodiments, the guardrings 190 may be replaced with a junction termination extension. It willalso be appreciated that the edge termination structure may be omittedin some embodiments.

When the power Schottky diode 100 is operated in reverse blocking mode,a strong electric field is formed that extends upwardly from thesubstrate 120 throughout the drift region 130 and toward and into thechannels 150. The electric field is reduced in the vicinity of the morehighly doped n-type local current spreading layers 160. As the localcurrent spreading layers 160 are provided below and adjacent thechannels 150, the intensity of the electric field may be reducedthroughout the channels 150. This reduction in the electric fieldintensity may result in lower reverse leakage currents.

As discussed above, the local current spreading layers 160 include alateral section 162 that extends laterally underneath a respective oneof the p-type blocking junctions 140, and one or more vertical sections164 that each extend along a sidewall of a p-type blocking junction 140.The vertical sections 164 are within the channels 150. In exampleembodiments, the vertical sections 164 may have a width W1 in thedirection of the widths of the channels 150 of less than 0.6 microns, asis shown in FIG. 3B. In other embodiments, the width W1 may be less than0.4 microns. In still other embodiments, the width W1 may be less than0.3 microns. In each of these embodiments, the width W1 may be at least0.05 microns in some example, cases, or may be at least 0.1 microns inother example cases. It will be appreciated, however, that the verticalsections 164 may have smaller or larger widths.

Since the middle portion of each channel 150 is not part of any localcurrent spreading layer 160, each channel 150 has a lower doped portionthat is doped, for example, at the same doping density as the remainderof the drift region 130. As discussed above, the ideal size of the gapW2 between adjacent blocking regions 140 (i.e., the widths of thechannels 150) may be a function of the doping density of the channel150. Since the majority of each channel 150 is doped at the same (lower)level as the drift region 130, the size of the gap W2 may be relativelylarge. For example, in some embodiments, a distance between the firstand second blocking junctions may be at least 1.5 microns. In otherembodiments, the distance between the first and second blockingjunctions may be at least 2.0 microns. In still other embodiments, thedistance between the first and second blocking junctions may be at least2.5 microns. In each of these embodiments, the distance between thefirst and second blocking junctions may be less than 5 microns, at leastin some cases. Implant masks for the ion implantation steps used to formthe blocking junctions 160 may be easily formed with gaps W2 this size,as the size of the gap W2 is much larger than the potential variation inthe masking process that may result from misalignment of the mask and/orunintended exposure of a photoresist due to bowing of the substrate.

The power Schottky diode 100 may exhibit improved performance ascompared to conventional power Schottky diodes. In particular, the V_(F)versus I_(R) performance for the diode 100 may shift the curve in themanner discussed above with reference to FIGS. 2A-2B so that improvedreverse leakage current performance may be obtained while maintaining adesired forward voltage level. The power Schottky diodes 100 mayconsistently be fabricated in a mass production setting as the inclusionof the local current spreading layers 160 makes the size of the gap W2relatively independent of the doping level of the local currentspreading layers 160.

FIG. 4 is a schematic cross-sectional view of a power Schottky diode 200according to additional embodiments of the present invention that hasboth local current spreading layers and a drift region having asuperjunction structure. As the power Schottky diode 200 is similar tothe power Schottky diode 100 discussed above with reference to FIGS.3A-3B, like elements in Schottky diode 200 are numbered using the samereference numerals, and will not be discussed further below as they havealready been described above. The discussion that follows will focus onthe differences between the power Schottky diode 200 and the powerSchottky diode 100.

As shown in FIG. 4, the power Schottky diode 200 includes a drift region230 that has a superjunction structure. In particular, the drift region230 includes regions 232 that are doped with n-type dopants and regions234 that are doped with p-type dopants. The regions 232, 234 may extendvertically through the drift region 230 and may have various shapes, isas known to those of skill in the art. The regions 232, 234 often havecolumn shapes or striped shapes and are typically referred to as n-typepillars 232 and p-type pillars 234. U.S. patent application Ser. No.15/168,310 (the '310 application), filed May 31, 2016, discloses variousexample superjunction structures that may be used in the power Schottkydiodes according to embodiments of the present invention. The entirecontent of the '310 application is incorporated herein by reference. Thepower Schottky diode 200 may have any of the superjunction structuresdisclosed in the '310 application, and the superjunction structure maybe formed by any of the techniques disclosed in the '310 application.

In some embodiments, the pillars 232, 234 may be formed using channeledion implantation techniques that are discussed in the '310 application.These channeled ion implantation techniques may allow the formation ofmore heavily doped pillars in the drift region 230 than can be achievedin conventional superjunction devices in a single ion implantation step(note that the single ion implantation step may include implantations atmultiple different energy levels). The drift region 230 may be grown toan increased thickness. The thicker drift regions increase the voltageblocking capabilities of the device, while the superjunction structuremay help reduce or eliminate any offsetting increase in the on-stateresistance of the power Schottky diode 200 that would otherwise occur asa result of the increased thickness of the drift layer 230.

While not shown in FIG. 4, it will be appreciated that two or moren-type pillars 232 and/or two or more p-type pillars 234 may be providedin each unit cell. The number of pillars 232, 234 provided will be afunction of the width selected for the pillars 232, 234. Typically, eachn-type pillar 232 and p-type pillar 234 will have the same width,although embodiments of the present invention are not limited thereto.The superjunction-type drift region 230 may be designed to be chargebalanced between the alternating n-type and p-type pillars 232, 234 insome embodiments. Additionally, while FIG. 4 shows the entirety of thedrift region 230 being implanted, it will be appreciated that this neednot be the case. For example, in other embodiments, only an upperportion of the drift region 230 may be implanted. It will also beappreciated that the doping concentration of the implanted portion ofthe drift region 230 may tend to decrease with increased distance fromthe upper surface of the device. Moreover, while in FIG. 4 thesuperjunction-type drift region 230 has n-type pillars 232 and p-typepillars 234 that appear to extend vertically from the substrate 120, itwill be appreciated that in other embodiments the pillars 232, 234 maybe at an oblique angle with respect to a top surface of the substrate120. Such angled pillars may be formed, for example, by forming thepillars 232, 234 using the channeled ion implantation techniques whichare discussed herein.

By providing both local current spreading layers 160 and a drift region230 having a superjunction structure, it is expected that improvedshielding of the channels 150 from the electric field that forms in thedrift region 230 during reverse blocking operation may be achieved,further shifting the V_(F) versus I_(R) performance curve down and tothe left as described in FIG. 2B above. It will be appreciated that eachof the power Schottky diodes according to embodiments of the presentinvention that are disclosed herein may have a drift region including asuperjunction structure, and that any of the superjunction structuresdisclosed in the '310 application and/or any other superjunctionstructure may be used in each of the different embodiments disclosedherein

The local current spreading layers 160 may be formed via ionimplantation. In some embodiments, the local current spreading layers160 may be formed using the channeled ion implantation techniques thatare disclosed in the '310 application. For example, as discussed in the'310 application, if the power Schottky diodes disclosed herein have a4H silicon carbide drift layer, the ion implantation may be performedalong one or more of the <11-23>, <−1-123>, <1-213>, <−12-13>, <2-1-13>and <−2113> crystallographic axes. Each of these six symmetricallyequivalent crystallographic axes are at a 170 tilt from the <0001>crystallographic axis. Along these six crystallographic axes, relativelylarge channels appear in the crystallographic structure that facilitatedeep implantation of dopants via ion implantation. In an alternativeembodiment, the ion implantation could be performed along the <11-20>crystallographic axis, which exhibits very large channels along with alow surface density of atoms as viewed along the axis of implantation.Likewise, the power Schottky diodes according to embodiments of thepresent invention that include superjunction structures in the driftregion may form some or all of the superjunction structures via thechanneled ion implantation techniques disclosed in the '310 application.The ability to form deep superjunction structures having relatively highdoping densities may significantly reduce the on-state resistance of thedevice, allowing for the use of thicker drift regions that may helpincrease the breakdown voltage. When channeled implants are used, theimplanted regions will be formed at an angle with respect to the topsurface of the drift layer.

The use of channeled ion implantation techniques may not only allow fordeeper implantation, but may also reduce the lateral distribution ofimplants, which provides better process control and smaller featuresizes. In addition, significantly thinner implant masks may be used whenchanneled ion implantation is performed, because the implant energiesmay be lower than would otherwise be required to achieve similar implantranges. In some cases, the implant mask may be less than half thethickness that would otherwise be required to obtain similar implantranges.

FIGS. 5A-5F are schematic cross-sectional diagrams that illustrate amethod of fabricating the power Schottky diode 100 according toembodiments of the present invention.

As shown in FIG. 5A, an n-type drift region 130 is epitaxially grown ona silicon carbide substrate 120. In some embodiments, the drift region130 may be doped during growth with n-type impurities to a concentrationof, for example, between 1×10¹⁵/cm³ and 5×10¹⁶/cm³. In this example, itis assumed that the drift region 130 is doped with n-type impuritiesduring growth to a concentration of 1×10¹⁶/cm³. Referring to FIG. SB,next, an ion implantation mask layer (not shown) may be formed on thedrift region 130, and this mask layer may then be patterned via, forexample, conventional photolithography processing steps to form an ionimplantation mask pattern 142. Subsequently, p-type dopants may beimplanted into the upper portions of the n-type drift region 130 thatare exposed by the ion implantation mask pattern 142 in order to formthe p-type blocking junctions 140. The p-type blocking junctions 140 maybe highly doped with p-type dopants (e.g., to a concentration of5×10¹⁸/cm³).

Referring to FIG. 5C, an etching process may be performed to etch theion implantation mask pattern 142 to provide an etched ion implantationmask 142′. In example embodiments, the ion implantation mask pattern 142may be an oxide mask pattern (e.g., a silicon oxide pattern). A bufferedoxide etch or other appropriate etching process may then be performedunder tightly controlled conditions in order to remove a predeterminedamount of material from the upper surface and exposed sidewalls of theion implantation mask pattern 142. Such an etching process may have ahigh degree of accuracy so that the amount of material removed from thesidewalls of the mask pattern 142 may be precisely controlled. Inexample embodiments, the amount of material removed from the sidewallsof the ion implantation mask pattern 142 in forming the etched ionimplantation mask 142′ may be between 0.2 and 0.5 microns of material,although different amounts may be removed in other embodiments.

Referring to FIG. 5D, a second ion implantation may be performed toimplant n-type dopants into the regions of the device exposed by theetched ion implantation mask 142′ to form the local current spreadinglayers 160. The n-type dopants may be implanted at a concentration of,for example, 1×10¹⁷/cm³. The n-type dopants may be implanted into thep-type blocking junction 140 and into the portions of the upper part ofthe drift region that are adjacent the p-type blocking junctions 140that are exposed by the etched mask pattern 142′. As the p-type blockingjunction 140 may be doped at a much higher doping concentration, theimplantation of the moderate n-type doping used to form the localcurrent spreading layers 160 does not material impact the doping of thep-type blocking junctions 140. The second ion implantation step that isused to form the local current spreading layers 160 may be designed toimplant the n-type dopants deeper into the device than the first ionimplantation step that is used to form the p-type blocking junctions140. Consequently, the local current spreading layers 160 may extendbeneath their respective p-type blocking junctions 140 to form a shieldagainst the electric field in the drift region 130 during reverseblocking operation.

It will also be appreciated that in other embodiments the p-typeblocking junctions 140 and the n-type local current spreading layers 160may be formed in the reverse order. For example, the ion implantationmask 142 shown in FIG. 5B may be formed to have wider openings (namelyopenings having the width shown in FIG. 5C), and n-type dopants may beimplanted through these openings to form the local current spreadinglayers 160. Then, an oxide layer may be conformally grown or depositedon the ion implantation mask to create a revised ion implantation maskhaving openings with the width illustrated in FIG. 5B. The p-typedopants may then be implanted through these openings to form the p-typeblocking junctions 140. Thus, it will be appreciated that the methodsdiscussed herein describe example methods for forming the power Schottkydiodes according to embodiments of the present invention, and that othermethods of forming these devices may be used.

As shown in FIG. 5E, the etched ion implantation mask pattern 142′ maythen be removed. Finally, as shown in FIG. 5F, the ohmic contact layer112, the cathode contact 110, the Schottky contact 170 and the anodecontact 180 may be formed to complete the power Schottky diode 100.

FIG. 6 is a flowchart that illustrates a method of fabricating a powerSchottky diode according to embodiments of the present invention. Asshown in FIGS. 5A and 6, an n-type drift layer may be formed on ann-type substrate (Block 300). As shown in FIGS. 5B and 6, first andsecond p-type blocking junctions may then be formed in respectivelocations in the upper portion of the drift region (Block 310). In someembodiments, the first and second p-type blocking junctions may beformed in the upper portion of the drift region by forming an ionimplantation mask pattern on an upper surface of the drift region andthen implanting p-type dopants into the upper portion of the driftregion through openings in the ion implantation mask pattern. As shownin FIGS. 5C, 5D and 6, first and second n-type local current spreadinglayers may then be formed underneath the respective first and secondblocking junctions (Block 320).

As shown in FIGS. 5C and 5D, in some embodiments, the first and secondn-type local current spreading layers may be formed underneath therespective first and second blocking junctions by partially etching theion implantation mask pattern to provide an etched ion implantation maskpattern and then implanting n-type dopants into the upper portion of thedrift region through openings in the ion implantation mask pattern toform the first and second local current spreading layers. The n-typedopants may be implanted deeper into the drift region than the p-typedopants that are implanted to form the first and second blockingjunctions. In some embodiments, a channeled ion implantation process maybe used to implant the n-type dopants that are implanted to form thefirst and second local current spreading layers. In one exampleembodiment, the drift region may be 4H silicon carbide, and thechanneled ion implantation may comprise implanting the n-type dopants atan angle that is within +/−1.5° of one of the <11-23>, <−1-123>,<1-213>, <−12-13>, <2-1-13> or <−2113> crystallographic axes of thedrift region.

As shown in FIG. 5D, in some embodiments, the first local currentspreading layer may surround a bottom and sidewalls of the firstblocking junction. In such embodiments, a portion of the first localcurrent spreading layer may be within the channel. A lateral width ofthe portion of the first local current spreading layer that is withinthe channel may be between about 0.1 and about 0.75 microns in exampleembodiments. In other example embodiments, the lateral width of theportion of the first local current spreading layer that is within thechannel may be between about 0.3 and about 0.6 microns. In still otherembodiments, the lateral width of the portion of the first local currentspreading layer that is within the channel may be between about 0.4 andabout 0.5 microns.

As shown in FIGS. 5F and 6, first and second contacts may then be formedon the respective upper and lower portions of the drift region (Block330). The first and second blocking junctions define a channel in theupper portion of the drift region, the channel doped with dopants havingthe first conductivity type and a concentration of dopants in at least afirst portion of the channel being lower than the concentration ofdopants in the first and second local current spreading layers.

While FIG. 6 describes formation of a power Schottky diode having ann-type drift layer, it will be appreciated that the conductivity type ofeach layer/region could be reversed in other embodiments.

FIGS. 7A-7C are graphs that illustrate the performance of the powerSchottky diodes according to embodiments of the present invention ofFIGS. 3A-3B as compared to (1) power Schottky diodes having full currentspreading layers (“full CSL”) in the upper portion of the drift regionand (2) optimized power Schottky diodes that do not include any currentspreading layer (“no CSL”). The power Schottky diodes having fullcurrent spreading layers comprise devices in which the upper portion ofthe drift region is grown with a higher doping concentration to form acontinuous current spreading layer. The p-type blocking junctions arethen formed in the continuous current spreading layer.

FIG. 7A illustrates the avalanche voltage of power Schottky diodeshaving local current spreading layers according to embodiments of thepresent invention as compared to full CSL and no CSL power Schottkydiodes. In FIG. 7A, each vertical column of data points representsindividual results for sample devices taken from different wafers. Thus,the length of each vertical column provides an indication as to theamount of process variation, with longer lengths indicating greateramounts of variation. Results are provided for both the full CSL and thelocal CSL power Schottky diodes having channel widths W2 of 2.0, 2.5,3.0, 3.5, 4.0 and 4.5 microns, and for the no CSL power Schottky diodeshaving channel widths W2 of 3.0, 3.5, 4.0 and 4.5 microns.

As shown in FIG. 7A, the local CSL power Schottky diodes exhibitimproved avalanche breakdown performance (which is used as an indicationof reverse blocking voltage) as compared to the full CSL power Schottkydiodes at all channel widths investigated, and provide comparableavalanche breakdown performance to the no CSL power Schottky diodes. Thelocal CSL and no CSL power Schottky diodes exhibit avalanche breakdownvoltages from about 2150-2300 volts, while the full CSL power Schottkydiodes exhibit avalanche breakdown voltages of 1700-2100 volts. Thelocal CSL power Schottky diodes exhibit performance variation that isless than half the variation seen in the full CSL power Schottky diodes,and process variation that is comparable to the no CSL power Schottkydiodes. Moreover, the local CSL power Schottky diodes (and the no CSLpower Schottky diodes) maintain consistent performance regardless of thechannel width, while the performance of the full CSL devices decreaseswith increasing channel width. Moreover, while not shown in FIG. 7A, thelocal CSL power Schottky diodes exhibit significantly less variation asa function of doping of the current spreading layer as compared to thefull CSL power Schottky diodes, which exhibit decreasing performancewith increasing doping concentrations.

FIG. 7B illustrates the reverse leakage current performance of the localCSL power Schottky diodes according to embodiments of the presentinvention as compared to no CSL and full CSL power Schottky diodes. Asshown in FIG. 71, the local CSL power Schottky diodes exhibitdramatically improved reverse leakage current performance at all channelwidths as compared to the full CSL power Schottky diodes, and alsoexhibit improved performance as compared to the no CSL power Schottkydiodes. The process variation for the local CSL devices is slightlyhigher than the full CSL power Schottky diodes and comparable to the noCSL power Schottky diodes. However, since the average reverse leakagecurrent for the local CSL power Schottky diodes is 1-2 orders ofmagnitude lower than the average reverse leakage current for the fullCSL power Schottky diodes, the increased process variation is notproblematic. The reverse leakage current performance of the local CSLpower Schottky diodes is dependent on the channel width, with smallerchannel widths providing improved performance.

FIG. 7C is a simplified scatter plot illustrating the reverse leakagecurrent and forward voltage performance of sample local CSL, full CSLand no CSL power Schottky diodes. In particular, instead of illustratingindividual data points as in a conventional scatter plot, FIG. 7C showsthree regions 400, 410, 420. Region 400 is the approximate boundaryshowing the V_(F)-I_(R) performance for a large number of samples offull CSL power Schottky diodes. As shown in FIG. 7C, these devicesexhibited relatively low forward voltage values, but relatively highreverse leakage currents. Region 410 is the approximate boundary showingthe V_(F)-I_(R) performance for a large number of samples of no CSLpower Schottky diodes. As shown in FIG. 7C, these devices exhibitedrelatively high forward voltage values, and relatively low reverseleakage currents, or essentially the opposite performance of the fullCSL devices. Region 420 is the approximate boundary showing theV_(F)-I_(R) performance for a large number of samples of local CSL powerSchottky diodes according to embodiments of the present invention. Asshown in FIG. 7C, these devices exhibited relatively low forward voltagevalues and relatively low reverse leakage currents. Thus, FIG. 7Cclearly illustrates that the local CSL power Schottky diodes accordingto embodiments of the present invention provide the best combination ofperformance in terms of low reverse leakage current and low forwardvoltage values.

The power Schottky diodes according to embodiments of the presentinvention may have relatively wide channel widths such as channel widthsof 2.0 microns or more. Devices having such channel widths may bereadily formed using conventional manufacturing techniques. The localcurrent spreading layers meanwhile may allow for lower reverse leakagecurrents and higher reverse blocking voltage while still meeting adesired forward voltage level. Moreover, the larger channel width mayresult in reduced process variation and may significantly improve deviceyields.

The power Schottky diodes according to embodiments of the presentinvention may exhibit a number of advantages as compared to conventionalpower devices. For example, as noted above, the power Schottky diodesaccording to embodiments of the present invention may exhibit lowerelectric field levels in the channels when the diodes are in theirreverse blocking state. As a result, the depletion regions of the unitcells may tend to merge at a deeper depth within the device layerstructure (i.e., toward the substrate) as compared to conventionaldevices. As a result, the power Schottky diodes according to embodimentsof the present invention may exhibit reduced reverse leakage current.The merging of these depletion regions may also provide enhanced voltageblocking capabilities as compared to conventional devices.

Additionally, the Schottky diodes according to embodiments of thepresent invention may also provide a non-destructive avalanche currentpath within an active area of the device. As known to those of skill inthe art, “avalanche breakdown” refers to a rapid current multiplicationthat can occur when a strong electric field is applied to the device. Inpower silicon carbide devices, much of this avalanche current willtypically flow through a termination region of the device that surroundsthe active area. Unfortunately, the termination regions of such devicestypically cannot handle avalanche current levels, and thus if avalanchebreakdown occurs, the device may be permanently destroyed. The localcurrent spreading layers included in the power Schottky diodes accordingto embodiments of the present invention may make it easier for avalanchecurrents to flow through the active area of the device, as the increaseddoping level under the blocking junctions results in an increasedelectric field at the p-n junction formed at the underside of theblocking junctions, which facilitates avalanche conditions beingreached. As a result, Schottky diodes according to embodiments of thepresent invention may include leakage current paths within the activearea of the device that carry the avalanche current when the devicebreaks down. When the avalanche current is carried through these leakagecurrent paths it may not destroy the device, and hence the Schottkydiodes according to certain embodiments of the present invention may bemore likely to survive avalanche events.

While the above embodiments of the present invention have primarily beendiscussed with reference to silicon carbide devices, it will beappreciated that the above techniques may also be used on other types ofpower Schottky diodes including power Schottky diodes fabricated ingallium nitride based materials.

While in the description above, the example embodiments are describedwith respect to semiconductor devices that have n-type substrates andchannels in n-type portions of the drift layers, it will be appreciatedthat opposite conductivity type devices may be formed by simplyreversing the conductivity of the n-type and p-type layers in each ofthe above embodiments. Thus, it will be appreciated that the presentinvention covers both n-type and p-type devices. Accordingly, the claimsappended hereto refer to the first and second conductivity type dopantsas opposed to n-type and p-type dopants. It will likewise be appreciatedthat typically each power semiconductor device formed according to theion implantation techniques disclosed herein will comprise a pluralityof individual devices that are disposed in parallel in a unit cellstructure.

Herein, embodiments of the present invention are typically describedwith respect to cross-sectional diagrams that only illustrate one or twounit cells of a power Schottky diode. It will be appreciated that actualimplementations will typically include a larger number of unit cells.However, it will also be appreciated that the present invention is notlimited to such devices, and that the claims appended hereto also coverpower Schottky diodes that comprise a single unit cell.

The local current spreading layers according to embodiments of thepresent invention may be implemented in vertical power Schottky diodesthat have a drift region that extends vertically between cathode andanode contacts. In such devices, the lateral direction refers to thehorizontal direction. Thus, the lateral width of a channel refers to thehorizontal width of the channel and may be measured as the distancebetween sidewalls of adjacent blocking junctions.

Embodiments of the present invention have been described above withreference to the accompanying drawings, in which embodiments of theinvention are shown. It will be appreciated, however, that thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth above. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

It will be understood that, although the terms first, second, etc. areused throughout this specification to describe various elements, theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. For example, a first elementcould be termed a second element, and, similarly, a second element couldbe termed a first element, without departing from the scope of thepresent invention. The term “and/or” includes any and all combinationsof one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”“comprising,” “includes” and/or “including” when used herein, specifythe presence of stated features, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, operations, elements, components, and/or groups thereof.

Relative terms such as “below” or “above” or “upper” or “lower” and thelike may be used herein to describe a relationship of one element, layeror region to another element, layer or region as illustrated in thefigures. It will be understood that these terms are intended toencompass different orientations of the device in addition to theorientation depicted in the figures.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” or extending “onto” anotherelement, it can be directly on or extend directly onto the other elementor intervening elements may also be present. In contrast, when anelement is referred to as being “directly on” or extending “directlyonto” another element, there are no intervening elements present. Itwill also be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present.

Embodiments of the present invention are described above with referenceto a flowchart. It will be understood that the operations specified incertain blocks of the flowchart may be carried out at the same timeand/or in an order different than shown in different embodiments.

Embodiments of the invention are described herein with reference to planand cross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the invention.The thickness of layers and regions in the drawings may be exaggeratedfor clarity. Additionally, variations from the shapes of theillustrations as a result, for example, of manufacturing techniquesand/or tolerances, are to be expected.

In the drawings and specification, there have been disclosed typicalembodiments of the invention and, although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation, the scope of the invention being set forth inthe following claims.

That which is claimed is:
 1. A Schottky diode, comprising: a driftregion having an upper portion and a lower portion, at least some of thedrift region doped with dopants having a first conductivity type; afirst blocking junction and a second blocking junction in the upperportion of the drift region, the first and second blocking junctionsdoped with dopants having a second conductivity type, the secondconductivity type being opposite the first conductivity type; a firstlocal current spreading layer underneath the first blocking junction anda second local current spreading layer underneath the second blockingjunction, the first and second local current spreading layers doped withdopants having the first conductivity type; a first contact on the upperportion of the drift region; a second contact on the lower portion ofthe drift region and vertically spaced apart from the first contact; anda channel in the upper portion of the drift region between the first andsecond blocking junctions, the channel doped with dopants having thefirst conductivity type and a concentration of dopants in at least afirst portion of the channel being lower than the concentration ofdopants in the first and second local current spreading layers.
 2. TheSchottky diode of claim 1, wherein the first local current spreadinglayer includes a lateral section that extends underneath the firstblocking junction and a vertical section that extends upwardly from thelateral section along a sidewall of the first blocking junction, thevertical section comprising part of the channel.
 3. The Schottky diodeof claim 2, wherein a width of the vertical section of the first localcurrent spreading layer is between 0.1 and 0.75 microns.
 4. The Schottkydiode of claim 1, wherein a distance between the first and secondblocking junctions is between 1.5 microns and 5.0 microns.
 5. TheSchottky diode of claim 1, wherein a doping concentration of at least aportion of the local current spreading layer exceeds a dopingconcentration of the first portion of the channel by at least a factorof five.
 6. The Schottky diode of claim 1, wherein a lateral width ofthe first blocking junction is approximately equal to a lateral width ofthe first current spreading layer.
 7. The Schottky diode of claim 1,wherein the drift region includes a superjunction structure havingalternating vertically extending regions of silicon carbide having therespective first and second conductivity types.
 8. The Schottky diode ofclaim 1, wherein the drift region, the first and second blockingjunctions and the first and second local current spreading layerscomprise silicon carbide, and wherein a substrate is interposed betweenthe lower portion of the drift region and the second contact.
 9. ASchottky diode, comprising: a drift region having an upper portion and alower portion, at least some of the drift region doped with dopantshaving a first conductivity type; a first blocking junction and a secondblocking junction in the upper portion of the drift region, the firstand second blocking junctions doped with dopants having a secondconductivity type, the second conductivity type being opposite the firstconductivity type; a first contact on the upper portion of the driftregion; a second contact and separated from the first contact along avertical axis; and a channel doped with dopants having the firstconductivity type in the upper portion of the drift region between thefirst and second blocking junctions, the channel having a non-uniformdoping concentration along a lateral cross-section thereof.
 10. TheSchottky diode of claim 9, further comprising a first local currentspreading layer underneath the first blocking junction and a secondlocal current spreading layer underneath the second blocking junction,the first and second local current spreading layers doped with dopantshaving the first conductivity type, the concentration of dopants in thefirst and second local current spreading layers being higher than theconcentration of dopants in the drift region.
 11. The Schottky diode ofclaim 10, wherein a vertical section of the first local currentspreading layer comprises a first side portion of the channel and avertical section of second local current spreading layer comprises asecond side portion of the channel that is opposite the first side ofthe channel, the vertical sections of the first and second local currentspreading layers having a higher doping concentration than a middlesection of the channel so that the channel has the non-uniform dopingconcentration along the lateral cross-section thereof.
 12. The Schottkydiode of claim 11, wherein a width of the vertical section of the firstlocal current spreading layer is between 0.1 and 0.75 microns.
 13. TheSchottky diode of claim 9, wherein a distance between the first andsecond blocking junctions is at least 1.5 microns.
 14. The Schottkydiode of claim 10, wherein a doping concentration of at least a portionof the local current spreading layer exceeds a doping concentration of amiddle section of the channel by at least a factor of five.
 15. TheSchottky diode of claim 10, wherein a lateral width of the firstblocking junction is less than a lateral width of the first localcurrent spreading layer.
 16. The Schottky diode of claim 10, wherein thedrift region includes a superjunction structure having alternatingvertically extending regions of silicon carbide having the respectivefirst and second conductivity types.
 17. The Schottky diode of claim 10,wherein the drift region, the first and second blocking junctions andthe first and second local current spreading layers comprise siliconcarbide.
 18. The Schottky diode of claim 1, wherein first and seconddoping concentrations of the respective first and second local currentspreading layers exceed a third doping concentration of the lowerportion of the drift region and a fourth doping concentration of anupper portion of the drift region that is below the first and secondlocal current spreading layers.
 19. The Schottky diode of claim 9,further comprising a substrate on the lower portion of the drift region,wherein the second contact is on the substrate opposite the driftregion.